Method of Manufacturing a Nickel Titanium Coil Actuator

ABSTRACT

A method for pre-loading a shape memory alloy coil spring wound in a first direction after a shape-setting treatment, the method comprising inverting the coil spring by attaching one end of the coil spring to a core and unwinding it while winding it around the core in a second direction opposite to the first direction.

INTRODUCTION

The invention described herein may be manufactured and used by, or for the Government of the United States for governmental purposes without the payment of any royalties thereon.

The present teachings related to a method for manufacturing a NiTi coil actuator having enhanced performance, for example by inverting the coil spring to pre-load the spring or annealing different sections of the coil spring at different temperatures.

BACKGROUND

Soft robotics is an emerging field that introduces a number of new challenges to roboticists. Soft robots will consist of a flexible body, possess an ability to deform their shape as dictated by various environments, and exhibit robustness upon impact. A challenging element of soft robotics is the soft actuator. Most conventional actuators contain rigid components that limit morphing ability; however, a soft actuator can deform along with the surrounding structure.

An exemplary candidate for use in a soft actuator is Nickel Titanium (NiTi), a shape memory alloy (SMA). NiTi's unique martensite transformation makes it inherently flexible. NiTi is also well known for its high energy density, despite its poor efficiency. NiTi has advantages in small scale implementations where other types of actuators are not available or exhibit poor energy density. The NiTi crystal lattice transforms from the martensite state to the austenite state and produces up to 4% length change as it is heated through its transition temperature range. To achieve larger stroke lengths from a small lattice structure alteration, NiTi can be restructured into coil springs.

SUMMARY

The present teachings provide method for pre-loading a shape memory alloy coil spring wound in a first direction after a shape-setting treatment, the method comprising inverting the coil spring by attaching one end of the coil spring to a core and unwinding it while winding it around the core in a second direction opposite to the first direction.

The present teachings also provide method for pre-loading a shape memory alloy coil spring after a shape-setting treatment, the method comprising inverting the coil spring by pulling the spring through its own center.

The present teachings also provide a method for controlling displacement of a shape memory alloy coil spring, the method comprising altering a transition temperature of discrete sections of the shape memory alloy coil spring, and applying two or more currents to the shape memory alloy coil spring, each of the two or more currents causing only certain portions of the shape memory alloy coil spring to contract.

The present teachings further provide a shape memory alloy coil spring configured for controlled displacement and comprising a first section of coil spring annealed at a first temperature, and a second section of coil spring annealed at a second temperature that is greater than the first temperature.

Additional objects and advantages of the present teachings will be set forth in part in the description which follows, and in part will be obvious from the description, or may be learned by practice of the present teachings. The objects and advantages of the present teachings will be realized and attained by means of the elements and combinations particularly pointed out in the appended claims.

Both the foregoing general description and the following detailed description are exemplary and explanatory only and are not restrictive of the present teachings, as claimed.

The accompanying drawings, which are incorporated in and constitute a part of this specification, illustrate an exemplary embodiment of the present teachings and, together with the description, serve to explain the principles of the present teachings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1(A)-(E) illustrates five representative states of an exemplary NiTi coil spring actuator.

FIG. 2 is a schematic diagram of a portion of an exemplary manufacturing device for NiTi coil springs.

FIG. 3 is a graph showing a correlation between permanent plastic deformation and annealing temperature of a NiTi coil spring.

FIG. 4 is a graph showing NiTi coil spring deflection in both an austenite state and a martensite state with various loads.

FIG. 5 illustrates an exemplary method for inverting a NiTi coil spring in accordance with the present teachings.

FIG. 6 illustrates another exemplary method for inverting a NiTi coil spring in accordance with the present teachings.

FIG. 7 shows a stretched NiTi coil spring, an unstretched NiTi coil spring, and an inverted loose-wound NiTi coil spring return.

FIG. 8 illustrates controlled displacement of multiple sections of NiTi coil spring, each section of NiTi coil spring having been annealed at different temperatures.

FIG. 9 shows input current as a marker of transition temperature of NiTi coils springs annealed at different temperatures.

DESCRIPTION OF THE EMBODIMENTS

Reference will now be made in detail to the present teachings, examples of which are illustrated in the accompanying drawings.

NiTi coil springs are typically made from NiTi wire that is wound into a coil shape, and which can be annealed at a high temperature to reset its ‘memorized’ shape. Annealing is also referred to as reshaping. The annealing temperature used in the reshaping process can affect the mechanical properties of NiTi and thus its actuation performance, including detwinning characteristics (the force at which twinned martensite begins to shift slightly along twinned boundaries to accommodate deformation) and loaded displacement performance. NiTi coil springs can undergo permanent deformation after weightlifting tests, the degree of permanent deformation being a function of the annealing temperature.

The present teachings provide a manufacturing process for NiTi coil spring actuators that can be used as micro-muscle fibers for soft robotics. The performance of the actuator is dependent not only on the diameters of both the coil and the wire, but also on the pitch of the coil. The present teachings provide a method for creating improved performance NiTi spring actuators by inverting an annealed spring, and a novel method for controlling displacement of NiTi coil springs. Discrete displacement control can be achieved in accordance with the present teachings by creating a single wire with segments annealed at different temperatures.

Although NiTi coil spring actuators are generally known, a model of the NiTi coil spring has not been fully developed. Most existing studies of NiTi coil springs refer to known mechanical spring equations and use two different shear moduli for the martensite and austenite phases, often denoted as G_(M) and G_(A). None of the existing NiTi coil spring studies document or discuss the change in free length of the coil spring induced by the Martensite to Austenite phase change. Referring to FIG. 1(A)-(E), the spring elongates to a new, longer free length X_(M0) when in the 100% detwinned martensite phase, compared to a shorter free length X_(A0) in the 100% austenite phase.

The strain of the NiTi coil spring can change due to two physical phenomena. The first physical phenomenon is the phase transformation of lattices between the martensite and austenite phases. The second physical phenomenon is the spring effect. The spring extends according to its spring constant, which is a function of various design parameters as well as the phase of the spring. The spring constant in the austenite phase is typically around 2-3 times larger than in the martensite phase, as can be seen in FIG. 1(A)-(E).

FIG. 1(A)-(E) shows five representative states describing the behavior of a NiTi coil spring during actuation. When the spring is heated above the austenite transition temperature without a load, it is contracted fully as shown in FIG. 1(A). When a load F is applied, it elongates by δ_(H) as shown in FIG. 1(B). When the temperature drops below the martensite transition temperature without an applied load, the crystal lattice twins without a noticeable global shape change as shown in FIG. FIG. 1(C)). FIG. 1(D) shows a state where the spring is cooled below the martensite transition temperature after a load has detwinned its crystal lattice. The free length of the spring increases by δ_(M) compared with the free length of the spring in its austenite phase. This displacement will occur only after a detwinning force is applied to a spring that has cooled down to a twinned martensite phase. FIG. 1(E) shows the spring at full martensite phase with a load F being applied. A displacement of δ_(L) is created with respect to the unloaded fully martensite phase state of FIG. 1(D).

A typical actuation cycle for a NiTi coil spring would begin with an initial load of F when the actuator is in the martensite phase. Thereafter, the coil can be activated to generate a displacement of δ_(effective) under a load of F. The effective displacement created by the spring for this case would be

δ_(effective)=δ_(M)+δ_(L)−δ_(H)  (1)

where δ_(M), δ_(L), and δ_(H) are illustrated in FIG. 1(A)-(E). Further, δ is a function of the load F, the spring coil diameter D, the NiTi wire diameter d, the number n of active coils in the spring, and the shear modulus G of the spring after annealing, and can be defined by the following equation.

$\begin{matrix} {\delta = \frac{8\; {FD}^{3}n}{{Gd}^{4}}} & (2) \end{matrix}$

The shear strain γ of the spring can be defined as

$\begin{matrix} {\gamma = \frac{\tau}{G}} & (3) \end{matrix}$

and the shear stress τ can be defined as

$\begin{matrix} {\tau = \frac{8{FD}\; \kappa}{\pi \; d^{3}}} & (4) \end{matrix}$

where κ is a stress correction factor using Wahl's formula as follows.

$\begin{matrix} {\kappa = {\frac{{4C} - 1}{{4\; C} - 4} + \frac{0.615}{C}}} & (5) \end{matrix}$

Spring index C is defined as

$\begin{matrix} {C = \frac{D}{d}} & (6) \end{matrix}$

The change in the free length of the spring when the spring changes from austenite phase to martensite phase δ_(M) can be calculated from Equation (2) through Equation (6).

$\begin{matrix} {\delta_{M} = \frac{{\pi\gamma}\; D^{2}n}{d\; \kappa}} & (7) \end{matrix}$

The shear strain γ of the spring as it changes from the austenite phase to the martensite phase is approximately 7%. Therefore, for a given F, the effective displacement δ_(effective) resulting from the NiTi coil spring changing from an austenite phase to a martensite phase can be calculated as

$\begin{matrix} {\delta_{effective} = {\frac{\pi \; \gamma \; D^{2}n}{d\; \kappa} + \frac{8{FD}_{eff}^{3}n}{G_{M}d^{4}} - \frac{8{FD}^{3}n}{G_{A}d^{4}}}} & (8) \end{matrix}$

The coil diameter D is reduced significantly when the spring is extended at the martensite phase. Therefore an effective spring diameter D_(eff) can be used for the displacement created by load F is

D_(eff)=D cos θ  (9)

where θ is the angle between the spring wire and the horizontal plane.

FIG. 2 illustrates a device for manufacturing micro NiTi coil springs. A core wire (having, for example, a 200 μm diameter) and a NiTi wire (e.g., a Dynalloy Flexnol® 100 μm diameter NiTi wire to be wound around the core wire) are clamped at a drill press. A weight can be attached at an end of the core wire as illustrated to keep tension in the core wire. A ball bearing can be utilized with the weight to prevent the weight from turning with the core wire, which can prevent instability during the winding process. A coil guide can be made of a metal tube that can slide along the core wire and a bar attached to the metal tube. The coil guide can keep the coiled NiTi wire tightly packed by applying upward forces and can keep the core wire vertical by canceling any tension caused by coiling the NiTi wire.

Once winding of the NiTi wire finishes, the end of the coil can be crimped with the core wire and baked in a furnace at a desired temperature.

The annealing temperature of the NiTi coil spring can change various parameters of the NiTi coil spring. Two noticeable trends have been identified that vary with annealing temperatures: (1) the detwinning force decreases as the annealing temperature increases; and (2) springs annealed at high temperatures tend to lose their designed dimensions.

Detwinning forces: One of the mechanical characteristics that changes as a function of the annealing temperature is the detwinning force. This can affect the work efficiency of the actuator, because the detwinning process requires external work input. NiTi requires a certain amount of force to return to the fully detwinned martensite state through martensite transformation.

External forces can be applied to a NiTi coil spring, for example by hanging different weights or applying different forces, to test the characteristics of the spring based on, for example, its annealing temperature. To minimize dynamic effects, a weight can be applied very slowly, and then, to eliminate spring effect, the weight can be removed slowly. The resulting elongated length of the NiTi coil spring is measured under no load.

Permanent deformation of NiTi coil springs: FIG. 3 shows permanently elongated lengths of NiTi coil springs annealed at various temperatures. The illustrated deformation statistics represent the difference between the length of a tight 25 mm spring in a martensite state and the length of the same spring in an austenite state after lifting a 35 g weight. The data shows deformation after one lift. The measured data with springs annealed below 400° C. shows reasonable consistency at multiple trials. The springs annealed at temperatures higher than 400° C. show more deformation. Although higher annealing temperatures can produce lower detwinning forces, for springs annealed above 400° C., the original dimension is lost after a single lifting cycle.

Based on these observations, for comparison of a model (as represented by the above equations) and an actual experiment in terms of stroke length, a spring annealed at 390° C. can be utilized for weight lifting experiments. FIG. 4 shows predicted and actual spring deflection in both an austenite state and a martensite state for various loads. The current used to maintain the spring in the austenite state for the results shown in FIG. 4 was 150 mA. Predicted values from Equation (2) are plotted on top of the experiment data. The shear modulus of the NiTi coils spring used for the estimated values is calculated through the spring's deflection length in the austenite state, because the shear modulus is a function of annealing temperature. The martensite phase shear modulus can be defined as one third of the austenite phase shear modulus, which is the typical case for NiTi.

The energy density of a NiTi coil spring can be obtained by measuring work done when it lifts a weight. A 25 mm (annealed length) NiTi coil spring annealed at 390° C. lifts a 37.5 g weight with a stroke length of 50 mm. The amount of work done is 0.0184 J and the weight of the actuator is 0.014 g. The measured energy density is 1226 J/kg, defined as the amount of work done in a single stroke of the actuator.

NiTi coil springs in accordance with the present teachings behave according to previously-defined Equations (2) and (8). Increasing the NiTi wire diameter or decreasing the diameter of the spring can increase the spring stiffness and decrease displacement of the spring. Altering these parameters is a very powerful way to modulate spring characteristics, but such power comes at the expense of fine tuning. For example, moving from a 100 μm wire to a 150 μm wire increases stiffness fivefold (see Equation (2)).

A much finer method for fine tuning NiTi coil spring characteristics involves varying the number of coils in a NiTi coil spring, either by altering the length of the spring (using more or fewer coils), or by altering spring coil pitch. To alter spring coil pitch, a spring can be coiled and annealed at a low temperature (e.g., 300° C.) to set the shape. Then the coil, while still wrapped around a core wire, can be stretched uniformly until the desired pitch is attained. Measurement of pitch is done through total spring length change, and calculated through simple geometry.

Varying the number of coils in a spring by cutting or pre-stretching is a very practical tuning method. To illustrate, if a 100 μm wire spring's stiffness needs to be increased by 10%, it can be achieved by simply pre-stretching 10%, as opposed to finding a new wire with a 105 μm diameter, as determined via Equation (2).

K. K. Jee et al., New method for improving properties of SMA coil springs, The European Physical Journal: Special Topics, 158:261-266, 2008, propose a method for creating an “initial tension” or pre-load in a shape memory alloy spring actuator by inverting it. Pre-loading can be useful for coil springs that require a compact spring and/or need to collapse the spring to an initial, unloaded coil length, even in the presence of a load. The traditional method of creating an initial tension on a spring requires twisting the wire while winding. Unfortunately, the subsequent shape-setting heat treatment necessary for shape memory alloy spring manufacturing relaxes the initial tension from twisting while winding, making this method ineffective.

Jee et al. disclose creating an initial tension by inverting the spring after the shape-setting treatment is complete. The mechanism for inverting the spring after heat treatment is not described, and has been interpreted as requiring careful bending using pliers. Such an inversion is only feasible for springs having a large spring index C and a short length. The present teachings provide methods for inverting springs of relatively small spring indexes and long lengths.

Two exemplary inversion methods in accordance with the present teachings are disclosed hereinbelow and are illustrated in FIGS. 5 and 6. A wrapping method illustrated in FIG. 6 comprises attaching one end of a formed coil spring to a core at X (e.g., the original core around which it was wound) and unwinding it while re-wrapping it around the core in the opposite direction. This method is suitable for coil springs having larger spring index values, but may be less practical for coil springs having smaller spring index values.

Another coil spring inversion method, illustrated in FIG. 6, comprises pulling the spring through its own center to invert it. In accordance with certain embodiments, to prevent the coils from getting tangled in each other, a thin-walled plastic tube can be inserted through the center of the coil spring. In certain embodiments, the inserted tube can also slightly expand the spring. In various embodiments of the present teachings, the spring can be initially wound around a core wire (see FIG. 2) and a length of its own winding wire, and the length of its own winding wire can be used as a center pull as illustrated in FIG. 6.

Both inversion methods can, for example, be used to invert springs having a spring index of C=10 and length of 2.5 cm. Both inversion methods can produce similar results in spring response. In certain embodiments of the present teachings, the original pre-inverted coil can be loosely wound, rather than tightly packed as in typical SMA spring actuator designs. In such a case, the spring wishes to return to the originally loose configuration after inversion, and therefore will have a quicker response time.

Testing of shape memory alloy coil springs inverted in accordance with the present teachings reveals that they have enhanced performance. For example, tests were performed using 100 μm shape memory alloy wire wound around an 8 mm core (spring index C=10). The coil springs were stretched using a 30 g mass and actuated with 6V at 0.55 A. Their response time and compression were recorded as set forth in Table 1 below.

TABLE 1 Compression Time Type Wrap (%) (s) Original Tight 62 10 Wrap Loose 77.81 7 Wrap Tight 70 10.5 Tube Loose 79.35 8 Tube Tight 74.36 12

FIG. 7 shows NiTi coil springs in accordance with the present teachings, each spring having the same diameter coil and wire, the same pitch, and the same composition. The NiTi coil spring pictured on the left is stretched by application of a weight W. The NiTi coil spring pictured in the center shows a resulting contraction of a tight-wound, un-inverted spring after application of the weight W. The NiTi coil spring pictured on the right shows a resulting contraction of a loose-wound, inverted spring after application of the weight W. Thus, the loose-wound, inverted NiTi coil spring contracts a greater distance after application of a weight.

Loose-wound inverted springs respond much faster and, as illustrated in FIG. 7, are able to retract almost to their original length because, upon inversion, the internal spring forces that originally made the spring extend instead make the spring compress. In accordance with various embodiments, the original un-inverted spring should be perfectly wound when performing the central tube inversion method, because imperfections in the winding pattern can cause the spring to become tangled while in the heated state, and thus not fully return to the stretched state after cooling.

NiTi is mostly used as a binary actuator, contracting completely once the martensite to austenite phase transition occurs. Controlling heat input to the NiTi coil spring can cause a partial phase transition and thus control displacement; however, such methods are very difficult to control because the inter-phase region is highly nonlinear and very narrow. Displacement control can, however, be achieved by altering the transition temperature of discrete sections of a NiTi coil spring. Annealing two springs at different temperatures has the effect of creating a slightly different transition temperature in each. Thus, by annealing a series of coils into a single coil, each at a slightly different annealing temperature, a coil spring can be created having sections of coil that contract at slightly different input temperatures.

FIG. 8 provides a partial view of a controlled displacement of multiple sections of NiTi coil spring, each section of NiTi coil spring having been annealed at different temperatures, as described below.

FIG. 9 shows input current as a marker of transition temperature for NiTi coil springs annealed at different temperatures. While the relationship is clearly nonlinear, there is a considerable difference in transition temperature such that a number of discrete coils can be made to actuate at discrete input currents. An example of controlled displacement is shown in FIG. 8, with three sections of NiTi coil spring annealed, respectively, at 370° C. (labeled as 800), 480° C. (labeled as 810), and 630° C. (labeled as 820). The coil spring sections exhibit sequential contractions as current is applied. In FIG. 9, more current is applied progressively as the illustrations move downward as indicated. Thus, at 0 Amps, coil spring sections 800, 810, and 820 have not contracted. At 0.115 Amps, coil spring section 800 (annealed at 370° C.) has contracted but coil spring sections 810 and 820 have not contracted. At 0.135 Amps, coil spring sections 800 (annealed at 370° C.) and 810 (annealed at 480° C.) have contracted but coil spring section 820 has not contracted. At 0.155 Amps, coil spring sections 800 (annealed at 370° C.), 810 (annealed at 480° C.), and 820 (annealed at 630° C.) have contracted. Thus, fine tuning of an overall length of shape memory alloy coil spring can be accomplished by annealing different sections of coil at different temperatures and actuating the coil spring at different amperages.

Other embodiments of the present teachings will be apparent to those skilled in the art from consideration of the specification and practice of the present teachings disclosed herein. It is intended that the specification and examples be considered as exemplary only, with a true scope and spirit of the present teachings being indicated by the following claims. For example, other shape memory allows can be used in addition to, or instead of NiTi. 

1. A method for pre-loading a shape memory alloy coil spring wound in a first direction after a shape-setting treatment, the method comprising inverting the coil spring by attaching one end of the coil spring to a core and unwinding it while winding it around the core in a second direction opposite to the first direction.
 2. The method of claim 1, wherein the shape memory alloy comprises NiTi.
 3. A method for pre-loading a shape memory alloy coil spring after a shape-setting treatment, the method comprising inverting the coil spring by pulling the spring through its own center.
 4. The method of claim 3, wherein the shape memory alloy comprises NiTi.
 5. The method of claim 3, further comprising inserting a plastic tube through the center of the coil spring and pulling the coil spring through the plastic tube to invert the coil spring.
 6. The method of claim 5, wherein the coil spring is initially wound around a core wire and a length of its own winding wire, and further comprising pulling the length of its own winding wire through the plastic tube to pull the spring through the plastic tube. 7-14. (canceled) 